The present invention relates to an image input apparatus having a CCD color image sensor.
JP-A-2003-163788 discloses an image input apparatus having a CCD color image sensor as an image device. The CCD color image sensor has, for each of the three primary colors: R (Red), G (Green) and B (Blue), plural photoelectric conversion elements 201 and shift registers 203 (FIG. 3). A photoelectric conversion element 201 has filters of the prescribed colors R, G and B. The photoelectric conversion element 201 receives light passed through the filter, and accumulates charge in accordance with the quantity of the received light. The charge accumulated in the photoelectric conversion element 201 is transferred to a shift register 203 by opening (enabling) a shift gate 202. The shift register 203 transfers the transferred charge to an output unit 204 using a charge-coupled device.
Such an image input apparatus having a CCD color image sensor (for example, an image scanner or a copying machine) reads an image recorded on a light-transmissive sheet such as a negative or positive film (hereinafter, referred to as a “transmissive original”), and an image recorded on a light-reflective sheet such as a printing or photographic paper (hereinafter, referred to as a “reflective original”).
A description is hereinafter given for a case in which a transmissive original such as a negative film is read. FIG. 8 is a timing diagram of pulses applied to open each shift gate 202 corresponding to each color of R, G and B (referred to as a “shift gate pulse”). To each shift gate 202, a shift gate pulse 301 is applied periodically so as to transfer charge to be extracted as an output signal 311 (referred to as “effective charge”).
The three primary colors are not equal in light transmission with a negative film, but in the order of B<G<R. Accordingly, accumulation time of the effective charge 311 is adjusted so as to be in the order of B>G>R. Timing of starting accumulation of the effective charge 311 is also set in the order of B, G and R so that the incident light quantities of the three primary colors are nearly equal. Specifically, a shift gate pulse 302 for discarding the charge accumulated before accumulating the effective charge 311 (referred to as an “unnecessary charge”) 312 is applied at different times in the order of B, G and R. This allows the accumulation time of the effective charge 311 to be adjusted in the order of B>G>R, thereby equalizing their incident light quantities.
When a shift gate pulse is applied with the timing shown in FIG. 8, however, unnecessary charge sometimes overflows in the photoelectric conversion element 201. FIG. 9 shows the quantity of the accumulated charge in the photoelectric conversion element 201 when a shift gate pulse is applied with the timing shown in FIG. 8. Note that a negative film to be read is assumed to contain the three primary colors in nearly equal proportions. As has been mentioned above, since the accumulation time of the effective charge 311 is adjusted thereby almost equalizing the incident light quantities, R, G and B have the same quantity of the accumulated effective charge (shown as “1” in FIG. 9). On the other hand, the quantity of the accumulated unnecessary charge 312 follows the order of R>G>B due to the influence of accumulation time and transmission (“3”, “1”, “⅓”, respectively in FIG. 9). This means that R has the largest quantity of accumulated unnecessary charge, with the fear that charge overflows if the quantity of the accumulated unnecessary charge exceeds the tolerable quantity of the charge accumulated in the photoelectric conversion element 201. If the tolerable quantity of the charge accumulated in the photoelectric conversion element 201 is less than 3 in FIG. 9, charge overflows. The charge overflowing in the photoelectric conversion element 201 has an influence on another pixel, causing smear.